In the fabrication of modern integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 .mu.m or less for high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process utilizing a fluorine chemistry is thus used for such via formation process.
The via hole formation process can be enhanced by improving the etch directionality by a mechanism known as sidewall passivation to improve the anisotropy of the etching process. By utilizing a suitable etchant gas and suitable reactor parameters, an etch-inhibiting film of a polymeric nature can be formed on vertical sidewalls. The etch-inhibiting film slows down or completely stops any possible lateral etching of horizontal surfaces in the via hole. For instance, when a fluorine-containing etchant gas such as CFH.sub.3 is used, a fluorine-type polymeric film is formed on the sidewalls. Many photoresist materials may also contribute to the formation of polymeric films on the sidewalls. After the sidewall is coated with a polymeric film, it is protected by the inhibitor film to preserve the line width or via hole diameter control.
In a modern etch chamber, an electrostatic chuck (or E-chuck), is frequently used in which the chuck electrostatically attracts and holds a wafer that is positioned on top. The use of E-chuck is highly desirable in the vacuum handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck can hold and move wafers with a force equivalent to several tens of Torr pressure. Another advantage for the E-chuck is that no particle generation or contamination problem can occur since there are no moving parts acting on the wafer. Moreover, the electrostatic force utilized on an E-chuck is sufficient in preventing bowing of a wafer which normally occurs in mechanical clamping and thus promotes uniform heat transfer over the entire wafer surface.
In an etch chamber equipped with a plasma generating device and an E-chuck, a shadow ring may be utilized as a seal around the peripheral edge of the wafer. The shadow ring, also known as a focus ring, is utilized for achieving a more uniform plasma distribution over the entire surface of the wafer and for restricting the distribution of the plasma cloud to only the wafer surface area. The uniform distribution function may be further enhanced by a RF bias voltage applied on the wafer during a plasma etching process. Another function served by the shadow ring is sealing at the wafer level the upper compartment of the etch chamber which contains the plasma from the lower compartment of the etch chamber which contains various mechanical components for controlling the E-chuck. This is important since it prevents the plasma from attacking the hardware components contained in the lower compartment of the etch chamber. In order to survive high temperature and hostile environments, a shadow ring is frequently constructed of a ceramic material such as quartz.
In an etch chamber equipped with a high density plasma and an E-chuck, problems sometimes arise in the operation of the E-chuck. High density gas plasma formed has a short debye length and consequently vary small sheaths are formed at boundaries of objects that are present in the gas plasma. The debye length is defined as the distance from the plasma at which the electron density drops to 1/e of the electron density in the bulk plasma. The debye length can be calculated by first dividing the electron temperature by the electron density, and then taking the square root. The typical electron temperature in a high density plasma is low, i.e., on the order of a few eV. On the other hand, the electron density is high, i.e., on the order of 10.sup.11 .about.10.sup.12 electrons per cubic centimeter. This leads to a small debye length on the order of approximately several tenths of a millimeter. Gaps found in a process chamber that is larger than a few debye length may either cause a gas breakdown or the plasma may be extracted into the sufficiently large gaps.
In order to prevent the plasma from affecting the voltage on the electrode of the E-chuck, the electrode positioned in a plasma chamber must be sufficiently isolated from the plasma. In a typical E-chuck positioned in a high density plasma, the electrode has a voltage applied to it with respect to a ground reference point. The wafer is referenced back to the same ground reference by the plasma. The effective voltage for the electrostatic clamping of the wafer is then the voltage which appears across the E-chuck dielectric layer between the isolated electrode and the wafer. The voltage applied to the isolated electrode may be positive or negative with respect to the chamber ground. However, the electrostatic force depends on the algebraic difference between the wafer and the isolated electrode.
When the gaps around an E-chuck exceed several debye lengths, plasma may either be generated in the gaps or may be extracted into the gaps. When the plasma contacts the electrostatic chuck which has an imperfect dielectric layer or the E-chuck electrode, a current may flow between the E-chuck and the plasma. The voltage at the E-chuck electrode is therefore affected. Typically, the magnitude of the E-chuck voltage is reduced when a current flows between the chuck and the plasma which leads to a reduction in the electrostatic force. The efficiency of the E-chuck for holding a wafer is therefore affected. Ideally, the solution to the problem is to shield the E-chuck from the high density plasma by limiting gaps between the E-chuck and a shadow ring around the E-chuck to less than several debye lengths. In such an ideal situation, plasma can be prevented from being generated in the gaps or being extracted into the gaps. Since the ideal equipment conditions cannot be achieved in a manufacturing environment, the generation of plasma in the gaps or the extraction of plasma into the gaps and therefore attacking a shadow ring which is normally fabricated of quartz cannot be avoided. In a normal fabrication environment, it has been found that a quartz shadow ring would only survive about one preventive maintenance cycle or about 2,000 wafers. The corrosion occurred on the surface of the quartz shadow ring is usually severe enough that it must be replaced during a preventive maintenance procedure.
Referring initially to FIG. 1, wherein a conventional etch chamber 10 which is equipped with a shadow ring 12 installed around an electrostatic chuck 16 is shown. The etch chamber 10 is equipped with a coil antenna 14 as a plasma source in a reaction chamber 20 formed by a silicon ceiling block 22, a dome-shaped sidewall 24, a chamber wall liner 26 and the electrostatic chuck 16. The dome-shaped sidewall 24 and the chamber wall liner 26 are normally fabricated of quartz. The chamber wall liner 26 may be equipped with an opening for the passage of a wafer paddle in loading and unloading wafers. It may be removed from the vacuum chamber 10 for cleaning.
The shadow ring 12 is positioned inside the plasma reaction chamber 20 which can be lifted up to a process position by positioning pins 32. The positioning pins 32 lift the shadow ring 12 away from the wafer when a wafer is being loaded or unloaded. A multiplicity of cooling gas channels 34 is provided inside the electrostatic chuck 16 at near its top surface 36. A high heat conductivity gas such as helium can be circulated through the cooling gas channels 34 to provide a suitable gas pressure on the bottom side of wafer 30 for transferring heat away from the wafer to the water-cooled E-chuck 16 during an etch process. The supply lines for the cooling gas to channel 34 are not shown. The electrostatic chuck 16 is aligned by an electrostatic chuck collar 38. The etching gas is fed into chamber 20 through gas inlet 28. A thermal coupler 42 is mounted in the silicon ceiling block 22 for controlling temperature.
A cross-sectional view of a simplified E-chuck and shadow ring construction is shown in FIG. 2. In this conventional structure, the E-chuck 16 is positioned inside a shadow ring 12 which may be fabricated of quartz. The E-chuck 12 is constructed in a slightly different shape than the E-chuck 16 shown in FIG. 1, i.e., in a L shape. The lower part 40 of the L is provided with a planar top surface 44 which supports an edge portion 18 of the wafer 30. During an etching process, gas plasma 46 bombards a top surface 48 of the wafer 30. The gap 52 which is normally formed between the shadow ring 12 and the wafer 30 permits the gas plasma 46 to enter the gap and thus attacking the surface 44 of the shadow ring 12. After repeated use, the surface 44 can be substantially corroded by the gas plasma. A substantially corroded top surface 44 of the shadow ring 12 leads to a gap formed between the end portion 18 of the wafer 30 and the lower part 40 of the shadow ring 12. Such gap formation (not shown in FIG. 2) allows the gas plasma 46 to enter the lower chamber of the etch chamber and to attack the E-chuck or the components that operates the E-chuck. The plasma attack can seriously damage the E-chuck and its associated components leading to expensive repair.
It is therefore an object of the present invention to provide a shadow ring for use in a plasma etch chamber that does not have the drawbacks or shortcomings of the conventional shadow rings.
It is another object of the present invention to provide a shadow ring that is constructed of an outer ring and an inner ring which are intimately mated together for sealing around a wafer positioned on a E-chuck.
It is a further object of the present invention to provide a composite shadow ring which is constructed by an outer ring and an inner ring in such away that only the inner ring is subjected to attack by the gas plasma in the etch chamber.
It is another further object of the present invention to provide a composite shadow ring for use in a plasma etch chamber wherein only a small inner ring portion is subjected to plasma attack which can be readily replaced at low cost.
It is still another object of the present invention to provide a composite shadow ring for use in a plasma etch chamber which includes an outer ring and an inner ring with the latter being a sacrificial ring that can be readily replaced.
It is yet another object of the present invention to provide a plasma etch chamber for processing a semiconductor substrate which includes a shadow ring for engaging an edge portion of the semiconductor substrate with only an inner ring portion of the shadow ring being substantially exposed to the gas plasma.
It is still another further object of the present invention to provide a method for conducting an etching process in a plasma etch chamber that is equipped with a composite shadow ring wherein an inner ring portion of the composite shadow ring is subjected to attack by the gas plasma and can be readily replaced during a preventive maintenance procedure.
It is yet another further object of the present invention to provide a method for conducting an etching process in a plasma etch chamber which is equipped with a composite shadow ring by exposing only an inner ring portion of the shadow ring to the gas plasma and then replacing such inner ring during each preventive maintenance procedure without the need for replacing he entire shadow ring.